My 1998 900S run fantastic until you come to a red light. While applying the brakes the engine seems to shudder (idle up and down). Now I have an automatic transmission so the idle should be constant regardless. I had the fuel pump changed last year for over 1,000 dollars. I really don't want to eat that expense again. Any ideas what this could be?

emissionwhiz, Thank You. The solution was perfect. Your advice also promted me to clean the MAC which did require a different cleaner also by CRC. Again thank you very much.emissionwhiz, Thank You. The solution was perfect. Your advice also promted me to clean the MAC which did require a different cleaner also by CRC. Again thank you very much.

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Re: My 1998 900S run fantastic until you come to a red...

Try the following it usually cure s this kind of problemHere is the most common cause of surges and stalls at stops and low erratic idle speed and rough idle, it is the idle speed control air-bypass valve and throttle valve (IAC for short), they get full of gunk over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake

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run a teat. Remove the vacuum line from the brake booster and plug off. Start engine and see if it idles normally. . Put your foot on the brake pedal and apply the brakes . If the idle is still normal replace the brake booster as there is an air leak in the diaphram.. The service light is probably recording misfiring and air fuel mixture problems.. run fault codes any way just to check it out .

1st to second sounds like a forced shift i would check the 1-2 shift solenoid inside the transmission and make sure the speed sensors are all in good working order, clean and apply electro grease to plugs on the transmission , as for the idle problem it could be many things from sensors for the engine to a torque converter lockup solenoid need more info is the check engine light on? you said it shudders is it missing or has a surging idle ( high and low) does it die when it goes low idle or stay running

I would have the wheel hubs/bearings checked for noise, resistance, or wear. This part is notorious for creating a shudder and all 4 wheels have them. If the shudder comes from the front it is probably just a bad hub. I have also had worn bushings cause this problem, but it's usually a combination of the bad hub and worn bushing.

If at idle you apply vacuum and it gets worse and shuts down or shutters, then that's not the problem. But, if you apply vacuum and there is a leak, there is a bad EGR valve on the car. If you apply vacuum and it operates the EGR and there is no change then the EGR should still be removed and the pintle cleaned, or a new EGR Valve should be installed.

Also, a coil pack could be causing this condition, where not enough spark gets to the spark plugs.

Is this a 4 tech or quad 4 engine?

Also, the cylinder head gasket could be burned through and allowing one cylinder to bleed exhaust to another cylinder on the Quad 4.

Hello! You have symptoms that point to incorrect fuel pressure.First have the fuel pressure tested as follows; #1...engine running at idle: 13- 20 psi. #2...key on engine off: 23-30 psi. If pressure is incorrect change the fuel pump.

Does the engine "surge" at idle? If so, check the "fuel injection pressure regulator idle air control valve". Apply a vacuum to it; If fuel flows it is not the problem.

I would Look For A Vacuum Leak, Given the Fact that ALL that Work was Done in a SHORT Amount of Time, And The Brakes ARE VACUUM ASSISTED, that when Brakes are applied the ENG. Light Comes on, Seems to Me It all May be related. It does not take Much to Pop A Vacuum line OFF Accidentally

when you are traveling about 35 or 40 mph press the brake just hard enough to make shudder let off do it again let and off. if you can control the shudder that way then its the brake if does not do it til you slow to a certian speed then the problem may be engine related. As far as the low idle there is an idle stabilizer which can definitely cause that problem. Or in some case an aged timing belt can cause similar issuses